The pattern of electrophysiological activation of adult mouse ventricles was measured with the use of voltage-sensitive dye methods. Di-4-ANEPPS was used to monitor membrane potential as small changes in fluorescence, which were detected by a state of the art, cooled, charged coupled device camera/image intensifier system. The extremely rapid conduction velocity, coupled with the small size of this preparation, necessitated taking these measurements at room temperature (22 to 23 degrees C). Initial experiments demonstrate that ventricular activation can be identified and its conduction pattern can be monitored reproducibly and with high resolution for extended time periods (10 to 20 mins) during spontaneous activity.